1. Field of the Invention
The present invention relates to a process for the preparation of austenitic stainless steel having an excellent corrosion resistance, especially seawater resistance. Furthermore, the present invention provides a steel material having an excellent workability such that edge cracking or face cracking does not occur when the material is hot-worked into a heavy plate, or a strip, or the like.
2. Description of the Related Art
The importance of stainless steel having a high corrosion resistance, especially a high resistance to corrosion from seawater, as the material for a plant for the desalination of seawater or the like will increase.
Most alloys suitable for use in this field contain Cr, Ni, Mo, Si and the like, and N is utilized as the element for improving the strength and corrosion resistance of stainless steel. As one such stainless steel material, the present inventors previously proposed a high-alloy stainless steel having not only a high corrosion resistance but also an excellent hot-workability, in Japanese Patent Application No. 60-4118 (Japanese Unexamined Patent Publication No. 61-163247).
Recently, a process is often adopted in which the step of forming a slab, as a material to be worked into a heavy plate or strip, from a high-alloy steel containing large quantities of elements as mentioned above, i.e., the step of forming a slab from a melt, is carried out by continuous casting. When a steel containing large quantities of Cr, Ni, Mo, and Si, is formed into a slab by continuous casting and the slab is hot-worked into a heavy plate or strip, an excellent workability is an important characteristic required for the production. At present, same technical problems must be solved, inclusive of this problem of the workability, in the production of high-alloy stainless steel materials by continuous casting.
As is well-known, Cr, Mo and N are especially important alloy components in stainless steel having a high resistance to corrosion from seawater, and it is particularly important that stainless steel having a high resistance to corrosion from seawater should contain 3 to 13% by weight of Mo.
Nevertheless, when a slab is formed by a continuous casting of 20% Cr-18% Ni type high-alloy steel containing 3 to 13% by weight of Mo, segregation having low contents of Mo and Cr is caused at the center in the thickness direction of the formed cast piece (slab), and it is impossible to obtain the aimed corrosion resistance in a final product because of this segregation.
Furthermore, the .sigma.-phase is precipitated at the cast piece-cooling step of the continuous casting process, and this .sigma.-phase is the factor that causes edge cracking or face cracking when the material is hot-worked.
As a means of improving the hot-workability by controlling the precipitation of the .sigma.-phase in the above-mentioned high-alloy cast piece or moderating the segregation of the alloy elements, the present inventors previously proposed a process in which a soaking (homogenizing treatment) of the cast piece is the main step (Japanese Patent Application No. 62-201028), but use of this technical means alone did not provide a sufficient resistance to corrosion from seawater.
A technical object of the present invention is to solve the problem of the impossibility of obtaining a good resistance to corrosion from seawater because of a segregation having low contents of alloy elements such as Mo and Cr at the center in the thickness direction of the slab, which occurs when preparing a slab by a continuous casting of the above-mentioned high-alloy steel. Another object of the present invention is to improve the hot-workability by eliminating the precipitation of the .sigma.-phase and to improve the corrosion resistance by diffusing Mo or Cr contained at a high content in the .sigma.-phase and eliminating Mo- or Cr-poor regions.